Tag: science

Something I’ve been thinking about a lot lately is how much information we really convey with emoji. I was recently at the 1​st​ International Workshop on Emoji Understanding and Applications in Social Media and one theme that stood out to me from the papers was that emoji tend to be used more to communicate social meaning (things like tone and when a conversation is over) than semantics (content stuff like “this is a dog” or “an icecream truck”).

I’ve been itching to apply an information theoretic approach to emoji use for a while, and this seemed like the perfect opportunity. Information theory is the study of storing, transmitting and, most importantly for this project, quantifying information. In other words, using an information theoretic approach we can actually look at two input texts and figure out which one has more information in it. And that’s just what we’re going to do: we’re going to use a measure called “entropy” to directly compare the amount of information in text and emoji.

What’s entropy?

Shannon entropy is a measure of how much information there is in a sequence. Higher entropy means that there’s more uncertainty about what comes next, while lower entropy means there’s less uncertainty. (Mathematically, entropy is always less than or the same as log2(n), where n is the total number of unique characters. You can learn more about calculating entropy and play around with an interactive calculator here if you’re curious.)

So if you have a string of text that’s just one character repeated over and over (like this: 💀💀💀💀💀) you don’t need a lot of extra information to know what the next character will be: it will always be the same thing. So the string “💀💀💀💀💀” has a very low entropy. In this case it’s actually 0, which means that if you’re going through the string and predicting what comes next, you’re always going to be able to guess what comes next becuase it’s always the same thing. On the other hand, if you have a string that’s made up of four different characters, all of which are equally probable (like this:♢♡♧♤♡♧♤♢), then you’ll have an entropy of 2.

TL;DR: The higher the entropy of a string the more information is in it.

Experiment

Hypothesis

We do have some theoretical maximums for the entropy text and emoji. For text, if the text string is just randomly drawn from the 128 ASCII characters (which isn’t how language works, but this is just an approximation) our entropy would be 7. On the other hand, for emoji, if people are just randomly using any emoji they like from the set of emoji as of June 2017, then we’d expect to see an entropy of around 11.

So if people are just using letters or emoji randomly, then text should have lower entropy than emoji. However, I don’t think that’s what’s happening. My hypothesis, based on the amount of repetition in emoji, was that emoji should have lower entropy, i.e. less information, than text.

Data

To get emoji and text spans for our experiment I used four different datasets: three from Twitter and one from YouTube.

I used multiple datasets for a couple reasons. First, becuase I wanted a really large dataset of tweets with emoji, and since only between 0.9% and 0.5% of tweets from each Twitter dataset actually contained emoji I needed to case a wide net. And, second, because I’m growing increasingly concerned about genre effects in NLP research. (Like, a lot of our research is on Twitter data. Which is fine, but I’m worried that we’re narrowing the potential applications of our research becuase of it.) It’s the second reason that led me to include YouTube data. I used Twitter data for my initial exploration and then used the YouTube data to validate my findings.

For each dataset, I grabbed all adjacent emoji from a tweet and stored them separately. So this tweet:

Love going to ballgames! ⚾🌭 Going home to work in my garden now, tho 🌸🌸🌸🌸

Has two spans in it:

Span 1: ⚾🌭

Span 2: 🌸🌸🌸🌸

All told, I ended up with 13,825 tweets with emoji and 18,717 emoji spans of which only 4,713 were longer than one emoji. (I ignored all the emoji spans of length one, since they’ll always have an entropy of 0 and aren’t that interesting to me.) For the YouTube comments, I ended up with 88,629 comments with emoji, 115,707 emoji spans and 47,138 spans with a length greater than one.

In order to look at text as parallel as possible to my emoji spans, I grabbed tweets & YouTube comments without emoji. For each genre, I took a number of texts equal to the number of spans of length > 1 and then calculated the character-level entropy for the emoji spans and the texts.

Analysis

First, let’s look at Tweets. Here’s the density (it’s like a smooth histogram, where the area under the curve is always equal to 1 for each group) of the entropy of an equivalent number of emoji spans and tweets.

Text has a much high character-level entropy than emoji. For text, the mean and median entropy are both around 5. For emoji, there is a multimodal distribution, with the median entropy being 0 and also clusters around 1 and 1.5.

It looks like my hypothesis was right! At least in tweets, text has much more information than emoji. In fact, the most common entropy for an emoji span is 0: which means that most emoji spans with a length greater than one are just repititons of the same emoji over and over again.

But is this just true on Twitter, or does it extend to YouTube comments as well?

The pattern for emoji & text in YouTube comments is very similar to that for Tweets. The biggest difference is that it looks like there’s less information in YouTube Comments that are text-based; they have a mean and median entropy closer to 4 than 5.

The YouTube data, which we have almost ten times more of, corroborates the earlier finding: emoji spans are less informative, and more repetitive, than text.

Which emoji were repeated the most/least often?

Just in case you were wondering, the emoji most likely to be repeated was the skull emoji, 💀. It’s generally used to convey strong negative emotion, especially embarrassment, awkwardness or speechlessness, similar to “ded“.

More info & further work

If you’re interested, the code for my analysis is available here. I also did some of this work as live coding, which you can follow along with on YouTube here.

For future work, I’m planning on looking at which kinds of emoji are more likely to be repeated. My intuition is that gestural emoji (so anything with a hand or face) are more likely to be repeated than other types of emoji–which would definitely add some fuel to the “are emoji words or gestures” debate!

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If you’ve been following my blog for a while, you may remember that last year I found that YouTube’s automatic captions didn’t work as well for some dialects, or for women. The effects I found were pretty robust, but I wanted to replicate them for a couple of reasons:

I only looked at one system, YouTube’s automatic captions, and even that was over a period of several years instead of at just one point in time. I controlled for time-of-upload in my statistical models, but it wasn’t the fairest system evaluation.

I didn’t control for the audio quality, and since speech recognition is pretty sensitive to things like background noise and microphone quality, that could have had an effect.

Speech Data

I used speech data from four varieties: the South (speakers from Alabama), the Northern Cities (Michigan), California (California) and General American. “General American” is the sort of news-caster style of speech that a lot of people consider unaccented–even though it’s just as much an accent as any of the others! You can hear a sample here.

For each variety, I did an acoustic analysis to make sure that speakers I’d selected actually did use the variety I thought they should, and they all did.

Systems

For the YouTube captions, I just uploaded the speech files to YouTube as videos and then downloaded the subtitles. (I would have used the API instead, but when I was doing this analysis there was no Python Google Speech API, even though very thorough documentation had already been released.)

Bing’s speech API was a little more complex. For this one, my co-author built a custom Android application that sent the files to the API & requested a long-form transcript back. For some reason, a lot of our sound files were returned as only partial transcriptions. My theory is that there is a running confidence function for the accuracy of the transcription, and once the overall confidence drops below a certain threshold, you get back whatever was transcribed up to there. I don’t know if that’s the case, though, since I don’t have access to their source code. Whatever the reason, the Bing transcriptions were less accurate overall than the YouTube transcriptions, even when we account for the fact that fewer words were returned.

Results

OK, now to the results. Let’s start with dialect area. As you might be able to tell from the graphs below, there were pretty big differences between the two systems we looked at. In general, there was more variation in the word error rate for Bing and overall the error rate tended to be a bit higher (although that could be due to the incomplete transcriptions we mentioned above). YouTube’s captions were generally more accurate and more consistent. That said, both systems had different error rates across dialects, with the lowest average error rates for General American English.

Differences in Word Error Rate (WER) by dialect were not robust enough to be significant for Bing (under a one way ANOVA) (F[3, 32] = 1.6, p = 0.21), but they were for YouTube’s automatic captions (F[3, 35] = 3.45,p < 0.05). Both systems had the lowest average WER for General American.Now, let’s turn to gender. If you read my earlier work, you’ll know that I previously found that YouTube’s automatic captions were more accurate for men and less accurate for women. This time, with carefully recorded speech samples, I found no robust difference in accuracy by gender in either system. Which is great! In addition, the unreliable trends for each system pointed in opposite ways; Bing had a lower WER for male speakers, while YouTube had a lower WER for female speakers.

So why did I find an effect last time? My (untested) hypothesis is that there was a difference in the signal to noise ratio for male and female speakers in the user-uploaded files. Since women are (on average) smaller and thus (on average) slightly quieter when they speak, it’s possible that their speech was more easily masked by background noises, like fans or traffic. These files were all recorded in a quiet place, however, which may help to explain the lack of difference between genders.

Neither Bing (F[1, 34] = 1.13, p = 0.29), nor YouTube’s automatic captions (F[1, 37] = 1.56, p = 0.22) had a significant difference in accuracy by gender.Finally, what about race? For this part of the analysis, I excluded General American speakers, since they did not report their race. I also excluded the single Native American speaker. Even with fewer speakers, and thus reduced power, the differences between races were still robust enough to be significant for YouTube’s automatic captions and Bing followed the same trend. Both systems were most accurate for Caucasian speakers.

As with dialect, differences in WER between races were not significant for Bing (F[4, 31] = 1.21, p = 0.36), but were significant for YouTube’s automatic captions (F[4, 34] = 2.86,p< 0.05). Both systems were most accurate for Caucasian speakers.While I was happy to find no difference in performance by gender, the fact that both systems made more errors on non-Caucasian and non-General-American speaking talkers is deeply concerning. Regional varieties of American English and African American English are both consistent and well-documented. There is nothing intrinsic to these varieties that make them less easy to recognize. The fact that they are recognized with more errors is most likely due to bias in the training data. (In fact, Mozilla is currently collecting diverse speech samples for an open corpus of training data–you can help them out yourself.)

A recent paper proposes using automatic speech recognition to automatically score a clinical test used to diagnose, among other things, speech impairment, ADHD, dementia and schizophrenia.

Voice recognition is increasingly being incorporated into the hiring process. One example is HireVue, a software product designed to pre-screen candidates before they talk to a recruiter. A direct quote from the HireVue CEO when asked whether it might make a mistake on a candidate: “the algorithm is always right. It would be a ‘Yes’ or a ‘No.’” (Yikes!)

Every automatic speech recognition system makes errors. I don’t think that’s going to change (certainly not in my lifetime). But I do think we can get to the point where those error don’t disproportionately affect already-marginalized people. And if we keep using automatic speech recognition into high-stakes situations it’s vital that we get to that point quickly and, in the meantime, stay aware of these biases.

If you’re interested in the long version, you can check out the published paper here.

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Which wavelength someone would use not to hear but feel it on the body as a vibration?

So this would depend on two things. The first is your hearing ability. If you’ve got no or limited hearing, most of your interaction with sound will be tactile. This is one of the reasons why many Deaf individuals enjoy going to concerts; if the sound is loud enough you’ll be able to feel it even if you can’t hear it. I’ve even heard stories about folks who will take balloons to concerts to feel the vibrations better. In this case, it doesn’t really depend on the pitch of the sound (how high or low it is), just the volume.

But let’s assume that you have typical hearing. In that case, the relationship between pitch, volume and whether you can hear or feel a sound is a little more complex. This is due to something called “frequency response”. Basically, the human ear is better tuned to hearing some pitches than others. We’re really sensitive to sounds in the upper ranges of human speech (roughly 2k to 4k Hz). (The lowest pitch in the vocal signal can actually be much lower [down to around 80 Hz for a really low male voice] but it’s less important to be able to hear it because that frequency is also reflected in harmonics up through the entire pitch range of the vocal signal. Most telephones only transmit signals between 300 Hz to 3400 Hz, for example, and it’s only really the cut-off at the upper end of the range that causes problems–like making it hard to tell the difference between “sh” and “s”.)

The takeaway from all this is that we’re not super good at hearing very low sounds. That means they can be very, very loud before we pick up on them. If the sound is low enough and loud enough, then the only way we’ll be able to sense it is by feeling it.

How low is low enough? Most people can’t really hear anything much below 20 Hz (like the lowest note on a really big organ). The older you are and the more you’ve been exposed to really loud noises in that range, like bass-heavy concerts or explosions, the less you’ll be able to pick up on those really low sounds.

What about volume? My guess for what would be “sufficiently loud”, in this case, is 120+ Db. 120 Db is as loud as a rock concert, and it’s possible, although difficult and expensive, to get out of a home speaker set-up. If you have a neighbor listening to really bass-y music or watching action movies with a lot of low, booming sound effects on really expensive speakers, it’s perfectly possible that you’d feel those vibrations rather than hearing them. Especially if there are walls between the speakers and you. While mid and high frequency sounds are pretty easy to muffle, low-frequency sounds are much more difficult to sound proof against.

Are there any health risks? The effects of exposure to these types of low-frequency noise is actually something of an active research question. (You may have heard about the “brown note“, for example.) You can find a review of some of that research here. One comforting note: if you are exposed to a very loud sound below the frequencies you can easily hear–even if it’s loud enough to cause permanent damage at much higher frequencies–it’s unlikely that you will suffer any permanent hearing loss. That doesn’t mean you shouldn’t ask your neighbor to turn down the volume, though; for their ears if not for yours!

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We’re all familiar with the sensation of sound so loud we can actually feel it: the roar of a jet engine, the palpable vibrations of a loud concert, a thunderclap so close it shakes the windows. It may surprise you to learn, however, that that’s not the only way in which we “feel” sounds. In fact, recent research suggests that tactile information might be just as important as sound in some cases!

What was that? I couldn’t hear you, you were touching too gently.I’ve already talked about how we cansee sounds, and the role that sound plays in speech perception before. But just how much overlap is there between our sense of touch and hearing? There is actually pretty strong evidence that what we feel can actually override what we’re hearing. Yau et. al. (2009), for example, found that tactile expressions of frequency could override auditory cues. In other words, you might hear two identical tones as different if you’re holding something that is vibrating faster or slower. If our vision system had a similar interplay, we might think that a person was heavier if we looked at them while holding a bowling ball, and lighter if we looked at them while holding a volleyball.

And your sense of touch can override your ears (not that they were that reliable to begin with…) when it comes to speech as well. Gick and Derrick (2013) have found that tactile information can override auditory input for speech sounds. You can be tricked into thinking that you heard a “peach” rather than “beach”, for example, if you’re played the word “beach” and a puff of air is blown over your skin just as you hear the “b” sound. This is because when an English speaker says “peach”, they aspirate the “p”, or say it with a little puff of air. That isn’t there when they say the “b” in “beach”, so you hear the wrong word.

Which is all very cool, but why might this be useful to us as language-users? Well, it suggests that we use a variety of cues when we’re listening to speech. Cues act as little road-signs that point us towards the right interpretation. By having access to a lots of different cues, we ensure that our perception is more robust. Even when we lose some cues–say, a bear is roaring in the distance and masking some of the auditory information–you can use the others to figure out that your friend is telling you that there’s a bear. In other words, even if some of the road-signs are removed, you can still get where you’re going. Language is about communication, after all, and it really shouldn’t be surprising that we use every means at our disposal to make sure that communication happens.

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Since I’m teaching Language and Society this quarter, this is a question that I anticipate coming up early and often. Accents–or dialects, though the terms do differ slightly–are one of those things in linguistics that is effortlessly fascinating. We all have experience with people who speak our language differently than we do. You can probably even come up with descriptors for some of these differences. Maybe you feel that New Yorkers speak nasally, or that Southerners have a drawl, or that there’s a certain Western twang. But how did these differences come about and how are perpetuated?

Clearly people have Accents because they’re looking for a nice little sub-compact commuter car.

First, two myths I’d like to dispel.

Only some people have an accent or speak a dialect. This is completely false with a side of flat-out wrong. Every single person who speaks or signs a language does so with an accent. We sometimes think of newscasters, for example, as “accent-less”. They do have certain systematic variation in their speech, however, that they share with other speakers who share their social grouping… and that’s an accent. The difference is that it’s one that tends to be seen as “proper” or “correct”, which leads nicely into myth number two:

Some accents are better than others. This one is a little more tricky. As someone who has a Southern-influenced accent, I’m well aware that linguistic prejudice exists. Some accents (such as the British “received pronunciation”) are certainly more prestigious than others (oh, say, the American South). However, this has absolutely no basis in the language variation itself. No dialect is more or less “logical” than any other, and geographical variation of factors such as speech rate has no correlation with intelligence. Bottom line: the differing perception of various accents is due to social, and not linguistic, factors.

Now that that’s done with, let’s turn to how we get accents in the first place. To begin with, we can think of an accent as a collection of linguistic features that a group of people share. By themselves, these features aren’t necessarily immediately noticeable, but when you treat them as a group of factors that co-varies it suddenly becomes clearer that you’re dealing with separate varieties. Which is great and all, but let’s pull out an example to make it a little clearer what I mean.

Imagine that you have two villages. They’re relatively close and share a lot of commerce and have a high degree of intermarriage. This means that they talk to each other a lot. As a new linguistic change begins to surface (which, as languages are constantly in flux, is inevitable) it spreads through both villages. Let’s say that they slowly lose the ‘r’ sound. If you asked a person from the first village whether a person from the second village had an accent, they’d probably say no at that point, since they have all of the same linguistic features.

But what if, just before they lost the ‘r’ sound, an unpassable chasm split the two villages? Now, the change that starts in the first village has no way to spread to the second village since they no longer speak to each other. And, since new linguistic forms pretty much come into being randomly (which is why it’s really hard to predict what a language will sound like in three hundred years) it’s very unlikely that the same variant will come into being in the second village. Repeat that with a whole bunch of new linguistic forms and if, after a bridge is finally built across the chasm, you ask a person from the first village whether a person from the second village has an accent, they’ll probably say yes. They might even come up with a list of things they say differently: we say this and they say that. If they were very perceptive, they might even give you a list with two columns: one column the way something’s said in their village and the other the way it’s said in the second village.

But now that they’ve been reunited, why won’t the accents just disappear as they talk to each other again? Well, it depends, but probably not. Since they were separated, the villages would have started to develop their own independent identities. Maybe the first village begins to breed exceptionally good pigs while squash farming is all the rage in the second village. And language becomes tied that that identity. “Oh, I wouldn’t say it that way,” people from the first village might say, “people will think I raise squash.” And since the differences in language are tied to social identity, they’ll probably persist.

Obviously this is a pretty simplified example, but the same processes are constantly at work around us, at both a large and small scale. If you keep an eye out for them, you might even notice them in action.

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So, quick review: understanding speechis hard to model and the first model we discussed, motor theory, while it does address some problems, leaves something to be desired. The big one is that it doesn’t suggest that the main fodder for perception is the acoustic speech signal. And that strikes me as odd. I mean, we’re really used to thinking about hearing speech as a audio-only thing. Telephones and radios work perfectly well, after all, and the information you’re getting there is completely audio. That’s not to say that we don’t use visual, or, heck, even tactile data in speech perception. The McGurk effect, where a voice saying “ba” dubbed over someone saying “ga” will be perceived as “da” or “tha”, is strong evidence that we can and do use our eyes during speech perception. And there’s even evidence that a puff of air on the skin will change our perception of speech sounds. But we seem to be able to get along perfectly well without these extra sensory inputs, relying on acoustic data alone.

This theory sounds good to me. Sorry, I’ll stop.Ok, so… how do we extract information from acoustic data? Well, like I’ve said a couple time before, it’s actually a pretty complex problem. There’s no such thing as “invariance” in the speech signal and that makes speech recognition monumentally hard. We tend not to think about it because humans are really, really good at figuring out what people are saying, but it’s really very, very complex.

You can think about it like this: imagine that you’re looking for information online about platypuses. Except, for some reason, there is no standard spelling of platypus. People spell it “platipus”, “pladdypuss”, “plaidypus”, “plaeddypus” or any of thirty or forty other variations. Even worse, one person will use many different spellings and may never spell it precisely the same way twice. Now, a search engine that worked like our speech recognition works would not only find every instance of the word platypus–regardless of how it was spelled–but would also recognize that every spelling referred to the same animal. Pretty impressive, huh? Now imagine that every word have a very variable spelling, oh, and there are no spaces between words–everythingisjustruntogetherlikethisinonelongspeechstream. Still not difficult enough for you? Well, there is also the fact that there are ambiguities. The search algorithm would need to treat “pladypuss” (in the sense of a plaid-patterned cat) and “palattypus” (in the sense of the venomous monotreme) as separate things. Ok, ok, you’re right, it still seems pretty solvable. So let’s add the stipulation that the program needs to be self-training and have an accuracy rate that’s incredibly close to 100%. If you can build a program to these specifications, congratulations: you’ve just revolutionized speech recognition technology. But we already have a working example of a system that looks a heck of a lot like this: the human brain.

So how does the brain deal with the “different spellings” when we say words? Well, it turns out that there are certain parts of a word that are pretty static, even if a lot of other things move around. It’s like a superhero reboot: Spiderman is still going to be Peter Parker and get bitten by a spider at some point and then get all moody and whine for a while. A lot of other things might change, but if you’re only looking for those criteria to figure out whether or not you’re reading a Spiderman comic you have a pretty good chance of getting it right. Those parts that are relatively stable and easy to look for we call “cues”. Since they’re cues in the acoustic signal, we can be even more specific and call them “acoustic cues”.

If you think of words (or maybe sounds, it’s a point of some contention) as being made up of certain cues, then it’s basically like a list of things a house-buyer is looking for in a house. If a house has all, or at least most, of the things they’re looking for, than it’s probably the right house and they’ll select that one. In the same way, having a lot of cues pointing towards a specific word makes it really likely that that word is going to be selected. When I say “selected”, I mean that the brain will connect the acoustic signal it just heard to the knowledge you have about a specific thing or concept in your head. We can think of a “word” as both this knowledge and the acoustic representation. So in the “platypuss” example above, all the spellings started with “p” and had an “l” no more than one letter away. That looks like a pretty robust cue. And all of the words had a second “p” in them and ended with one or two tokens of “s”. So that also looks like a pretty robust queue. Add to that the fact that all the spellings had at least one of either a “d” or “t” in between the first and second “p” and you have a pretty strong template that would help you to correctly identify all those spellings as being the same word.

Which all seems to be well and good and fits pretty well with our intuitions (or mine at any rate). But that leaves us with a bit of a problem: those pesky parts of Motor Theory that are really strongly experimentally supported. And this model works just as well for motor theory too, just replace the “letters” with specific gestures rather than acoustic cues. There seems to be more to the story than either the acoustic model or the motor theory model can offer us, though both have led to useful insights.

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Ok, so, a couple weeks ago I talked about why speech perception was hard to model. Really, though, what I talked about was why building linguistic models is a hard task. There’s a couple other thorny problems that plague people who work with speech perception, and they have to do with the weirdness of the speech signal itself. It’s important to talk about because it’s on account of dealing with these weirdnesses that some theories of speech perception themselves can start to look pretty strange. (Motor theory, in particular, tends to sound pretty messed-up the first time you encounter it.)

The speech signal and the way we deal with it is really strange in two main ways.

The speech signal doesn’t contain invariant units.

We both perceive and produce speech in ways that are surprisingly non-linear.

So what are “invariant units” and why should we expect to have them? Well, pretty much everyone agrees that we store words as larger chunks made up of smaller chunks. Like, you know that the word “beet” is going to be made with the lips together at the beginning for the “b” and your tongue behind your teeth at the end for the “t”. And you also know that it will have certain acoustic properties; a short break in the signal followed by a small burst of white noise in a certain frequency range (that’s a the “b” again) and then a long steady state for the vowel and then another sudden break in the signal for the “t”. So people make those gestures and you listen for those sounds and everything’s pretty straightforwards right? Weeellllll… not really.

It turns out that you can’t really be grabbing onto certain types of acoustic queues because they’re not always reliably there. There are a bunch of different ways to produce “t”, for example, that run the gamut from the way you’d say it by itself to something that sound more like a “w” crossed with an “r”. When you’re speaking quickly in an informal setting, there’s no telling where on that continuum you’re going to fall. Even with this huge array of possible ways to produce a sound, however, you still somehow hear is at as “t”.

And even those queues that are almost always reliably there vary drastically from person to person. Just think about it: about half the population has a fundamental frequency, or pitch, that’s pretty radically different from the other half. The old interplay of biological sex and voice quality thing. But you can easily, effortlessly even, correct for the speaker’s gender and understand the speech produced by men and women equally well. And if a man and woman both say “beet”, you have no trouble telling that they’re saying the same word, even though the signal is quite different in both situations. And that’s not a trivial task. Voice recognition technology, for example, which is overwhelmingly trained on male voices, often has a hard time understanding women’s voices. (Not to mention different accents. What that says about regional and sex-based discrimination is a topic for another time.)

And yet. And yet humans are very, very good a recognizing speech. How? Well linguists have made some striking progress in answering that question, though we haven’t yet arrived at an answer that makes everyone happy. And the variance in the signal isn’t the only hurdle facing humans as the recognize the vocal signal: there’s also the fact that the fact that we are humans has effects on what we can hear.

Ooo, pretty rainbow. Thorny problem, though: this shows how we hear various frequencies better or worse. The sweet spot is right around 300 kHz or so. Which, coincidentally, just so happens to be where we produce most of the noise in the speech signal. But we do still produce information at other frequencies and we do use that in speech perception: particularly for sounds like “s” and “f”.
We can think of the information available in the world as a sheet of cookie dough. This includes things like UV light and sounds below 0 dB in intensity. Now imagine a cookie-cutter. Heck, make it a gingerbread man. The cookie-cutter represents the ways in which the human body limits our access to this information. There are just certain things that even a normal, healthy human isn’t capable of perceiving. We can only hear the information that falls inside the cookie cutter. And the older we get, the smaller the cookie-cutter becomes, as we slowly lose sensitivity in our auditory and visual systems. This makes it even more difficult to perceive speech. Even though it seems likely that we’ve evolved our vocal system to take advantage of the way our perceptual system works, it still makes the task of modelling speech perception even more complex.